DC current is the unidirectional flow of electric charge. Traditionally, in undergraduate physics courses, DC current problem solving involves the application of formulas to simple electrical circuits where the flow of charge takes place within metallic conductors. The new MCAT places less emphasis on this type of problem solving, but you still need to master it. Not only because many devices from the medical and life science research context rely on DC current technology, such as electrophoresis apparatus, but also because, like so much with physics, problem solving with the model system is the first step before applying concepts to real world situations. On the new MCAT you are more likely to find situations where charge carriers are ions moving through a solution or across a membrane than to be faced with electrons moving through a metal wire, but to interpret current in the life science context you need to first master the language of voltage, current, resistance, and power to describe circuits composed of wires, resistors, and voltage sources.

Ohm's lawOhm's law states that, in an electrical circuit, the current passing through a conductor between two points is proportional to the potential difference across the two points, and inversely proportional to the resistance between them.

Electrical conductorConductors, such as copper or aluminum, are materials with atoms having loosely held valence electrons.

Voltage dropVoltage drop is the reduction in voltage in an electrical circuit between the source and load.

Voltage sourceA voltage source is any device or system that produces an electromotive force between its terminals OR derives a secondary potential from a primary source of the electromotive force.

Kirchhoff's circuit lawsKirchhoff's circuit laws are a pair of laws that deal with the conservation of charge and energy in electrical circuits.

Electrical conductionConduction is the movement of electrically charged particles through a transmission medium which can form an electric current in response to an electric field.

Electrical conductivityElectrical or specific conductivity is a measure of a material's ability to conduct an electric current.

ResistivityElectrical resistivity is a measure of how strongly a type of material opposes the flow of electric current.

VoltmeterA voltmeter is a very high resistance ammeter used for measuring the electrical potential difference between two points in an electric circuit.

OhmmeterAn ohmmeter is an electrical instrument that measures electrical resistance, the opposition to the flow of an electric current.

SemiconductorA semiconductor is a solid that has electrical conductivity in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically.

Free electron modelIn solid-state physics, the free electron model is a simple model for the behaviour of valence electrons in a crystal structure of a metallic solid.

Current sourceA current source is an electrical or electronic device that delivers or absorbs flow of electric charge.

Wheatstone bridgeA Wheatstone bridge is a device used to measure an unknown electrical resistance which works by balancing two legs of a bridge circuit, one leg of which includes the unknown component.

SuperconductivitySuperconductivity is a phenomenon occurring in certain materials at extremely low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field.

Bridge circuitA bridge circuit is a type of electrical circuit in which the current in a conductor splits into two parallel paths and then recombines into a single conductor, thereby enclosing a loop.

Current densityCurrent density is a vector whose magnitude is the electric current per cross-sectional area measured in the SI system as amperes per square meter.

PotentiometerA potentiometer is a variable resistor that can be used as a voltage divider.

Advanced terms that may appear in context in MCAT passages

Drift velocityThe drift velocity is the average velocity, not speed, that a particle, such as an electron, attains due to an electric field.

Voltage dividerA voltage divider is a simple device designed to create a voltage which is proportional to another voltage.

Electronic band structureThe electronic band structure of a solid describes ranges of energy that an electron is forbidden or allowed to have.

N-type semiconductorAn N-type semiconductor is obtained by carrying out a process of doping with valence-five elements to a valence-four semiconductor in order to increase the number of free, negative charge carriers.

P-type semiconductorA P-type semiconductor is obtained by carrying out a process of doping the semiconductor to increase the number of free, positive charge carriers.

ExcitonAn exciton is a bound state of an electron and an imaginary particle called an electron hole in an insulator or semiconductor.

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